1. Field of the Invention
The present invention relates to an active matrix type display device and driving method thereof in which a transistor and a light emitting element are provided in each pixel and a current source circuit can control light emission of a pixel by each transistor. More particularly, the present invention relates to an active matrix type EL display device using an electroluminescence element as a light emitting element.
2. Description of the Related Art
An active matrix EL display device having a transistor which control a light emitting element and light emission of the light emitting element has been gathering attention in recent years. Such display device has advantages of superiority in response, operation with a low voltage, a wide view angle and the like. Therefore, the active matrix display device has attracted attention as a next generation flat panel display.
Driving methods in the case where a multi-level gray scale image is displayed using a light emitting device provided with a light emitting element, are broadly divided into an analog gray scale method and a digital gray scale method. A difference between both the methods is a method of controlling the light emitting element in respective states of light emission and non-light emission of the light emitting element. The former analog gray scale method is a method of controlling the amount of current flowing into the light emitting element to obtain gray scale in analog form. The latter digital gray scale method is a method of driving the light emitting element with only two states of an ON state (state in which luminance is substantially 100%) and an OFF state (state in which luminance is substantially 0%).
The driving methods are broadly divided into a current input method and a voltage input method depending on the type of video signals (image signals) input to a pixel provided with a light emitting element. The current input method is a method performed by signal current, whereas the voltage input method is a method controlled by voltage.
Next, an example of a circuit structure of a pixel that adopts a current input method and a driving method thereof in a display device will briefly be explained with reference to FIG. 18. A pixel shown in FIG. 18 has a signal line 1801, first to third scanning lines 1802 to 1804, a power source line 1805, transistors 1806 to 1809, a capacitor element 1810, and a light emitting element 1811. A current source circuit 1812 is provided for the signal line.
A gate electrode of the transistor 1806 is connected to the first scanning line 1802. A first electrode of the transistor 1806 is connected to the signal line 1801 whereas a second electrode thereof is connected to a first electrode of the transistor 1807, a first electrode of the transistor 1808 and a first electrode of the transistor 1809. A gate electrode of the transistor 1807 is connected to the second scanning line 1803. A second electrode of the transistor 1807 is connected to a gate electrode of the transistor 1808. A second electrode of the transistor 1808 is connected to the power source line 1805. A gate electrode of the transistor 1809 is connected to the third scanning line 1804. A second electrode of the transistor 1809 is connected to one of electrodes of the light emitting element 1811. The capacitor element 1810 is connected between the gate electrode and the second electrode of the transistor 1808 to hold the gate-source voltage of the transistor 1808. The power source line 1805 and a cathode of the light emitting element 1811 receive predetermined electric potentials to hold an electric potential difference, respectively.
Operations from writing video signal to light emitting will be described below. First, pulses are inputted to the first scanning line 1802 and the second scanning line 1803 to turn the transistors 1806 and 18070N. Signal current (video signal) flowing in the signal line 1801 at this point is denoted by Idata and the Idata is supplied from the current source circuit 1812.
Accumulation of electric charges in the capacitor element 1810 continues until the electric potential difference between its two electrodes, namely, the gate-source voltage (VGS) of the transistor 1808, reaches a desired voltage, that is, a voltage high enough to cause the current Idata to flow into the transistor 1808. When the accumulation of electric charges is finished, the signal current Idata continues to flow into the transistor 1808. A signal setting operation is conducted as above. Lastly, the selection of the first scanning line 1802 and the second scanning line 1803 is completed and the transistors 1806 and 1807 are turned to be OFF.
A light emission operation is described as follows. A pulse is inputted to the third scanning line 1804 to turn the transistor 1809 ON. With the transistor 1808 turned ON by VGS that is accumulated in the capacitor element 1810 in the preceding operation, current flows from the power source line 1805 to cause the light emitting element 1811 to emit light. Therefore, even if a characteristic of the transistor 1808 is unstable, the operation is not influenced.
Other pixel circuit structures that adopt current input method have been reported in U.S. Pat. No. 6,229,506 and Japanese Patent Laid Open No. 2001-147659.
In the structure of pixel circuit for current input method, a current value of signal current (video signal) written into a pixel and a driving current value when a light emitting element emits light need to be almost the same. However, an accurate setting operation needs much time, unless the current value of the signal current (video signal) is set to high value, because a parasitic capacitance (e.g., a capacitance at intersection of wirings) or resistance of wirings are generated in signal lines or the like provided in a pixel portion of a display device. On the contrary, the driving current value for causing a light emitting element to emit light is very small. Therefore, it is difficult to perform an accurate setting operation. Further, a variation in electric characteristic of the transistor 1808 is generated. This causes a fluctuation of current flowing to a light emitting element and a display unevenness.
Further, in displaying by analog gray scale method, it is required to write signal current having intensity corresponding to gray scale into each pixel every time a display is performed in each pixel. Thus, there is a necessity that electric charges corresponding to the signal current must be held again in a capacitor portion (a storage capacitor) of each pixel. It is difficult to perform an accurate setting operation when the signal current supplied to a pixel is small, that is, luminance is small. Further, an influence of a noise of a leak current or the like which occurs from a plurality of pixels connected to the same source signal line is tremendous, from other than the pixel in which writing of the signal current is carried out. On that account, there is such a high risk that it is impossible to cause the pixel to emit light with accurate luminance.
Accordingly, it is a distinct constraint that a current value of a video signal and a driving current value are the same value.
On the contrary, in controlling a transistor as a switch in digital form by a voltage input method (digital gray scale method), a constant voltage is applied to a light emitting element in a state of light emission. However, a relationship between a flowing current and a voltage applied to a light emitting element is changed due to a temperature in surrounding environment or deterioration of a light emitting element. Even if a constant voltage is applied to both electrodes of a light emitting element, a flowing current is actually changed. As a result, burning-in in a display or the like occurs.